Work to date has strongly suggested that the control of cell division in non-transformed (3T3) and transformed (3T3) mammalian cells comes at least partially from circulating macromolecules that have exquisite selectivity in their actions. The factor that is most limiting for the growth of 3T3 cells does not limit the growth of SV-3T3 cells. In a similar manner the factor that is depleted by growing SV-3T3 cells and limits the growth of a variety of transformed cells does not limit the growth of 3T3 cells. These observations allow for the theoretical possibility that the growth of malignant cells may be impeded without altering the non-transformed cell to a significant degree. The goal of the proposed research is to isolate and to characterize the factor(s) in serum which malignant cells require for growth.